Turning to FIGS. 1 and 2, an example of a conventional LINC transmitter 100 can be seen. In operation, a input signal S(t) (which has a varying envelope) is provided to the signal generator 102, which can be represented as:S(t)=A(t)eiθ(t),  (1)where A(t) is the signal envelope and θ(t) is the signal phase. The signal generator 102 is then able to generate signals S1(t) and S2(t) from signal S(t), which can be represented as:
                                                                        S                ⁡                                  (                  t                  )                                            =                            ⁢                                                1                  2                                ⁢                                  (                                                                                    S                        1                                            ⁡                                              (                        t                        )                                                              +                                                                  S                        2                                            ⁡                                              (                        t                        )                                                                              )                                                                                                        =                            ⁢                                                c                  2                                ⁢                                  (                                                            ⅇ                                              ⅈ                        ⁡                                                  (                                                                                    θ                              ⁡                                                              (                                t                                )                                                                                      +                                                          φ                              ⁡                                                              (                                t                                )                                                                                                              )                                                                                      +                                          ⅇ                                              ⅈ                        ⁡                                                  (                                                                                    θ                              ⁡                                                              (                                t                                )                                                                                      -                                                          φ                              ⁡                                                              (                                t                                )                                                                                                              )                                                                                                      )                                                                                                        =                            ⁢                              c                ⁢                                                                  ⁢                                                      ⅇ                                          ⅈθ                      ⁡                                              (                        t                        )                                                                              (                                                                                    ⅇ                                                  ⅈφ                          ⁡                                                      (                            t                            )                                                                                              +                                              ⅇ                                                  -                                                      ⅈφ                            ⁡                                                          (                              t                              )                                                                                                                                            2                                    )                                                                                                                        =                                ⁢                                  c                  ⁢                                                                          ⁢                                      ⅇ                                          ⅈθ                      ⁡                                              (                        t                        )                                                                              ⁢                                      cos                    (                                          φ                      ⁡                                              (                        t                        )                                                              )                                                              ,                                                          (        2        )            where c is radius shown in FIG. 2 and φ(t) is the out-phasing angle. When combining equations (1) and (2) and solving for the out-phasing angle φ(t), it becomes:
                              φ          ⁡                      (            t            )                          =                              arccos            ⁡                          (                                                A                  ⁡                                      (                    t                    )                                                  c                            )                                .                                    (        3        )            Since the arccosine function is limited to a domain between −1 and 1, then:c≧max(A(t)),  (4)which means that the signals S1(t) and S2(t) have a generally constant envelope. As a result, high-efficiency, nonlinear power amplifiers (PAs) can be used as PAs 104-1 and 104-2 to generate signals O1(t) and O2(t), which can then be combined with combiner 106 to produce signal O(t) that has a variable envelope.
One issue with LINC transmitter 100 is that there is an efficiency loss (due in part to combiner 106), so, as an alternative, an Asymmetric Mutlilevel Outphasing (AMO) transmitter 200 can be employed, as shown in FIG. 3. In operation, the AMO modulator 202 (which generally includes predistortion that is adjusted by the predistortion trainer 212) generates amplitude signals AMP-1 and AMP-2 and phase signals φ-1 and φ-2 from input amplitude signal AMP and input phase signal φ. The phase signals φ-1 and φ-2 are provided to the digital-to-radio-frequency phase converter (DRFPC) 204 that produces generally constant envelope signals for PAs 208-1 and 208-2 (similar to signals generator 102), and the amplitude signals AMP-1 and AMP-2 are used to control the power level applied to PAs 208-1 and 208-2 from supplies 206-1 and 206-2 to achieve higher efficiency. As shown in FIG. 4, the power is switched in regions where the probability distribution function (PDF) is the largest. This allows the AMO transmitter 200 to have greater overall efficiency than the conventional LINC transmitter 100 and a multilevel LINC (ML-LINC) transmitter but less overall efficiency than an power added efficiency (PAE). Because the efficiency of the AMO transmitter 200 is still relatively low, however, there is a need for an RF transmitter with improved efficiency.
Some examples of conventional circuits are: Chung et al. “Asymmetric Multilevel Outphasing Architecture for Multi-standard Transmitters,” 2009 IEEE Radio Frequency Integrate Circuits Symposium, pp. 237-240; Godoy et al., “A Highly Efficient 1.95-GHz, 18-W Asymmetric Multilevel Outphasing Transmitter for Wideband Applications,” Microwave Symposium Digest (MTT), 2011 IEEE MTT-S International, Jun. 5-10, 2011, pp. 1-4; U.S. Pat. No. 6,366,177; and U.S. Pat. No. 7,260,157.